The present invention relates to a combined packing-tray used in a vapor-liquid contacting tower, wherein a structure packing layer is located in the space under the vapor-liquid bubble contacting tray, a downcomer extends downward through the tray, the packing layer is located at the waist of the outside of the downcomer, and also to a process for effecting operation with high capacity and high turndown ratio by using the combined packing-tray.
Vapor-liquid contacting towers are used to separate special components from a multi-component mixture. Generally speaking, the vapor-liquid contacting towers employ trays, packings or their combination. The combination of random or structure packings and trays facilitates to improve the separation efficiency of components in the mixture. In recent years, new development has been made in the combined technique of tray and packing.
G. X. Chen, A. Afacan, C. Xu and K. T. Chuang proposed a combination of metal knitted mesh packing with conventional sieve tray, the packing being put on the tray [The Canadian Journal of Chemical Engineering, Vol. 68, June, 1990]. The packing acts for emulsifying the liquid, uniformizing the bubbles on the tray deck, and enlarging the vapor-liquid contacting area and thereby raising the tray efficiency, but it is less effective for enhancing the capacity through the tray.
Kejian Yao, Jianbing Ji, et al. discloses a method for combining structure packing under the dual-flow tray, which offers primary advantages of enhancing the high efficiency film mass transfer zone and improving the mass transfer efficiency in the bubbling zone on the tray deck. It is a highly efficient combination [Chemical Engineering, Vol. 20, No. 6, Dec. 25, 1992]. However, the turndown ratio and load capacity are limited due to its low liquid capacity of the dual-flow tray.
U.S. Pat. No. 5,366,666 discloses a combined random packing under a rectangular multiple downcomer tray (abbreviated as MD tray). However, it was testified in a heat simulation unit by UOP that it is hard for this combined tray to offer uniform fluid distribution. This structure needs to be improved.
U.S. Pat. No. 5,262,094 to K. T. Chuang et al. discloses a combined structure of packing and a dual-flow tray. The combined packing-tray is limited in industry application because the dual-flow tray has its inherent disadvantages of small liquid capacity and low turndown ratio. In an attempt to enhance the capacity and the turndown ratio, Chuang proposes employing a dual-flow tray with downcomer to allow 25%-70% of the liquid to flow downward through the opening of the tray deck onto the adjacent tray deck below and the remaining liquid to flow downward through the downcomer. However, in order to ensure that 25-70% of the total liquid weeps down through the opening of the tray deck, the opening proportion in the tray deck must be still very large, so that the vapor speed through the opening is far lower than the vapor speed at the weeping point, i.e., the stability coefficient (the ratio of the operating vapor speed through the opening to the vapor speed at the weeping point) is below 0.5. If the tower operates under conditions of a very low stability coefficient, the operating stability of the tower is very poor. Once the fluctuation of the load in practice causes the variation of the liquid weeping through the opening of the tray deck, the liquid entering into the downcomer follows to variate. When the load of the gas increases, the amount of the liquid weeping through the opening of the tray deck will drop immediately and the split-flow liquid through the dowacomer will increase sharply such that the liquid can not flow downward through the downcomer and flooding will take place. In the case of a low load of the gas, the amount of the liquid flowing through the downcomer increases and the split-flow through the downcomer is so small that the downcomer can not self-seal. The vapor ascends through the downcomer, and there is no liquid layer on the tray deck, resulting in reduced mass transfer efficiency. Thus, the combined packing-tray with a downcomer as defined in U.S. Pat. No. 5,262,094 can not have a higher capacity and a higher turndown ratio.
U.S. Pat. No. 5,407,605 to Resetarits et al. discloses another combined packing-dual-flow tray. 25-40 vol % of the total liquid weeps through the opening of the tray deck into the packing. Triangular or rectangular downcomers are installed on the tray deck to split the liquid flow. This is also a combined packing-tray based on a dual-flow tray with downcomers. It also has the defects similar to those of U.S. Pat. No. 5,262,094 to Chuang that the design of the liquid split by the dual-flow tray and the downcomer can not meet the requirement of the practice and the capacity and turndown ratio can not be enhanced.
In brief, there is inherent defect of unstable operation for all the combined packing-trays based on the dual-flow tray and thus it is difficult to meet the requirements of high capacity and high turndown ratio. The present invention aims for overcoming the deficiencies existing in the prior art.
The object of the present invention is to provide a new combined packing-tray to enable it to offer high capacity and high turndown ratio during operation.
The present invention is accomplished through the following technical solution.
One object of the present invention is to provide a combined packing-tray in a vapor-liquid contacting tower comprising:
a tray deck;
a downcomer or several downcomers extending through said tray deck downward; and
a structure packing layer located in the vapor phase space under said tray deck;
wherein at certain vapor-liquid load the structural dimensions of said tray deck and said downcomer should be determined such that at least 90 vol % of the total liquid flowing across the tray deck flows downward through said downcomer to the adjacent tray deck below.
Another object of the present invention is to provide a process for effecting an operation with high capacity, high turndown ratio and high separation efficiency in a vapor-liquid contacting tower comprising:
Providing a combined packing-tray in a vapor-liquid contacting tower which comprises a tray deck; a downcomer or several downcomers extending through the tray deck downward; and a packing layer located in the vapor phase space under the tray deck; wherein at certain vapor-liquid load the structural dimensions of the tray deck and the downcomer should be determined such that at least 90 vol % of the total liquid flowing across the tray deck flows downward through the downcomer to the adjacent tray deck below.
Subjecting the liquid to flowing from the top to the bottom of the tower, the liquid descending from the downcomer of the adjacent tray deck above, traversing the present tray deck and entering into the downcomer of the present tray deck, and substantially no liquid weeping downward through the opening of the tray deck; and
Subjecting the vapor to flowing from the bottom to the top of the tower, the vapor ascending through the opening of the tray deck, coming in contact with the liquid flowing horizontally on the tray deck to form a froth so that the contact of the vapor and the liquid on the tray deck is cross-flow bubble contact;
wherein the entrained droplets are removed and the vapor flow is distributed uniformly by action of barrier packing layer when the vapor flows upward through the froth so as to improve mass transfer efficiency and increase operating upper limit and operating turndown ratio.